Ultrasonics is the science of the effects of sound vibrations beyond the limit of audible frequencies. The object of high power ultrasonic applications is to bring about some permanent physical change in the material treated. This process requires the flow of vibratory energy per unit of area or volume. Depending on the application, the resulting power density may range from less than a watt to thousands of watts per square centimeter. Although the original ultrasonic power devices operated at radio frequencies, today most operate at 20,000-69,000 Hz.
Ultrasonics is used in a wide variety of applications. For example, ultrasonics can be used for dust, smoke and mist precipitation; preparation of colloidal dispersions; cleaning of metal parts and fabrics; thermoplastic bonding; the formation of catalysts; the degassing and solidification of molten metals; metal welding; the extraction of flavor oils in brewing; electroplating; drilling hard materials; fluxless soldering; and nondestructive testing such as in diagnostic medicine.
Ultrasonic force is used to perforate sheet materials. U.S. Pat. No. 3,966,519 discloses perforating non-woven webs. The ultrasonic energy amount is controlled by applying a fluid to the area where the ultrasonic energy is applied. U.S. Pat. No. 3,949,127 discloses perforating non-woven webs by applying intermittent ultrasonic fusion to the web and then stretching the web to break the most intensely fused regions causing perforations to form in the web. U.S. Pat. No. 5,269,981 discloses perforating a thin nonporous film which requires applying a liquid to the film before subjecting it to ultrasonic vibrations. U.S. Pat. No. 5,735,984 discloses forming apertures in an adhesive coated non-woven or foam sheet where the height of the flattened raised areas on the anvil is equal to or less than the thickness of the sheet material and adhesive.
Ultrasonic forces are used in welding sheet materials. U.S. Pat. No. 3,697,357 discloses welding sheets made entirely or partially of thermoplastic material or fiber by sealing an area of material. U.S. Pat. No. 3,939,033 discloses using ultrasonics to simultaneously seal and cut thermoplastic textile material. U.S. Pat. No. 5,061,331 discloses an ultrasonic cutting and edge sealing apparatus for cutting and sealing semi-permeable and at least partially thermoplastic fabric.
In acoustic bonding or welding, such as ultrasonic welding, two parts to be joined (typically thermoplastic parts) are placed directly below an ultrasonic horn. In plunge bonding or welding, the horn plunges (travels toward the parts) and transmits ultrasonic vibrations into the top part. The vibrations travel through the top part to the interface of the two parts. Here, the vibrational energy is converted to heat due to intermolecular friction that melts and fuses the two parts. When the vibrations stop, the two parts solidify under force, producing a weld at the joining surface.
Continuous ultrasonic welding is typically used for sealing fabrics, films, and other parts. In the continuous mode, typically the ultrasonic horn is stationary and the part is moved beneath it. Scan welding is a type of continuous welding in which the plastic part or web material is scanned beneath one or more stationary horns. In rotary horn welding, a rotating horn is used typically in conjunction with a rotating anvil. In traverse welding, both the table over which the parts pass and the part being welded remain stationary with respect to each other while moving underneath the horn or while the horn moves over them.
Many uses of ultrasonic energy for bonding and cutting thermoplastic materials involve ultrasonic horns. A horn is an acoustical tool usually having a length of a multiple of one-half of the horn material wavelength and made of, for example, aluminum, titanium, or steel that transfers the mechanical vibratory energy to the part. (Typically, these materials have wavelengths of approximately 25 cm (10 in).) Horn displacement or amplitude is the peak-to-peak movement of the horn face. The ratio of horn output amplitude to the horn input amplitude is termed gain. Gain is a function of the ratio of the mass of the horn at the vibration input and output sections. Generally, in horns, the direction of amplitude at the face of the horn is coincident with the direction of the applied mechanical vibrations.
Traditionally, ultrasonic cutting and welding use horns which vibrate axially against a rigid anvil, with the material to be welded or cut being placed between the horn and anvil. Alternatively, in continuous high speed welding or cutting, the horn is stationary while the anvil is rotated, and the part passes between the horn and the anvil. In these cases, the linear velocity of the part is matched with the tangential velocity of the working surface of the rotating anvil.
However, drag between the part and the horn can cause stress in and around the weld area during welding. Additionally, closure or nip forces also create stress in the bond area. These factors affect the weld quality and strength which, in turn, limit the line speeds.
One way to minimize these limitations is to shape the working surface of the horn to attain a progressive convergent or divergent gap depending upon the part. This can reduce the stresses but does not eliminate the drag stress. A carrier web can virtually eliminate drag stress.
Another way to attain high quality and high speed ultrasonic welds is to use a rotary horn with a rotating anvil. This system can reduce or eliminate the drag stress during the weld. Typically, a rotary horn is cylindrical and rotates around an axis. The input vibration is in the axial direction and the output vibration is in the radial direction. The horn and anvil can be two cylinders close to each other, rotating in opposite directions with substantially equal tangential velocities. The part to be bonded passes between these cylindrical surfaces at a linear velocity which equals the tangential velocity of these cylindrical surfaces. Matching the tangential velocities of the horn and the anvil with the linear velocity of the material can minimize the drag between the horn and the material. The excitation in the axial direction is similar to that in conventional plunge welding.
U.S. Pat. No. 5,096,532 describes two classes of rotary horn. The patent compares a commercially available rotary horn, manufactured by Mecasonic of Annemasse France (Mecasonic horn) and a rotary horn described in the '532 patent. The shape of the '532 horn differs from that of the Mecasonic horn; the '532 horn is solid, and the Mecasonic horn is a partially hollowed cylinder.
The Mecasonic horn is a full wavelength horn. The axial vibration excites the cylindrical bending mode to provide the radial motion, and the mode of vibration depends on Poisson's ratio. The radial motion of the weld face is in phase with the excitation, and there are two nodes for the axial motion, and two nodes for radial motion. The '532 horn is a half wavelength horn. The axial vibration provides the radial motion. The mode of vibration is independent of Poisson's ratio. The radial motion of the weld face is out of phase with the excitation, and there is only one node, at the geometric center of the weld face.
U.S. Pat. Nos. 5,707,483 and 5,645,681 describe novel rotary acoustic horns.
In known systems, welding is governed by the various parameters which can be changed to alter the welding process. These parameters include the frequency, amplitude of vibration, the duration of ultrasonic exposure, temperature (which is a function of the frequency), and the pressure between the horn and the anvil.
To properly weld materials, it is necessary to raise the interfacial temperature of the item being welded to allow the operation, such as bonding, to occur. It is also necessary to quench it and reduce the temperature rapidly, after the weld is made, to prevent stress from damaging the bonded area while the bond is still warm. The speed at which the temperature can be increased and decreased is often the limiting factor in the speed of welding.